Invasion and metastasis are the most insidious and life-threatening aspects of cancer. While tumors with minimal or no invasion may be successfully removed, once the neoplasm becomes invasive, it can disseminate via the lymphatics and/or vascular channels to multiple sites, and complete removal becomes very difficult. Invasion and metastases kill hosts through two processes: local invasion and distant organ colonization and injury. Local invasion can compromise the function of involved tissues by local compression, local destruction, or prevention of normal organ function. The most significant turning point in cancer, however, is the establishment of distant metastasis. The patient can no longer be cured by local therapy alone at this point.
The process of metastasis is a cascade of linked sequential steps involving multiple host-tumor interactions. This complex process requires the cells to enter into the vascular or lymphatic circulation, arrest at a distant vascular or lymphatic bed, actively extravasate into the organ interstitium and parenchyma, and proliferate as a secondary colony. Metastatic potential is influenced by the local microenvironment, angiogenesis, stroma-tumor interactions, elaboration of cytokines by the local tissue, and by the molecular phenotype of the tumor and host cells.
Local microinvasion can occur early, even though distant dissemination may not be evident or may not yet have begun. Tumor cells penetrate the epithelial basement membrane and enter the underlying interstitial stroma during the transition from in situ to invasive carcinoma. Once the tumor cells invade the underlying stroma, they gain access to the lymphatics and blood vessels for distant dissemination while releasing matrix fragments and growth factors. General and widespread changes occur in the organization, distribution, and quantity of the epithelial basement membrane during the transition from benign to invasive carcinoma.
Therapeutic efforts in cancer prevention and treatment are being focused at the level of signaling pathways or selective modulatory proteins. Protein kinase activities, calcium homeostasis, and oncoprotein activation are driving signals and therefore may be key regulatory sites for therapeutic intervention. Kinases in signaling pathways regulating invasion and angiogenesis may be important regulators of metastasis. One of the largest classes of biochemical molecular targets is the family of receptor tyrosine kinases (RTKs). The most common receptor tyrosine kinase molecular targets to date are the EGF and vascular endothelial growth factor (VEGF) receptors. Newer kinase molecular targets include the type III RTK family of c-kit, and abl. Inhibitors of these molecules have been administered in combination with classic chemotherapeutics.
Metastases ultimately are responsible for much of the suffering and mortality from cancer. A need exists to identify and target molecular and functional markers that identify metastatic cancer cells and to generate reagents for their specific inhibition.
Publications in this field include, inter alia, Li et al. Oncogene. (2009) 28(39):3442-55; United States Patent Application, 20050186571 by Ullrich et al.; United States Patent Application 20080293733 by Bearss et al.; Sun et al. Oncology. 2004; 66(6):450-7; Gustafsson et al. Clin Cancer Res. (2009) 15(14):4742-9; Wimmel et al. Eur J Cancer. 2001 37(17):2264-74; Koorstra et al. Cancer Biol Ther. 2009 8(7):618-26; Tai et al. Oncogene. (2008) 27(29):4044-55
The receptor tyrosine kinase AXL (also known as Ufo and Tyro7) belongs to a family of tyrosine receptors that includes Tyro3 (Sky) and Mer (Tyro12). A common ligand for AXL family is GAS6 (Growth arrest-specific protein 6). Human AXL is a 2,682-bp open reading frame capable of directing the synthesis of an 894-amino acid polypeptide. Two variant mRNAs have been characterized, transcript variant 1 may be accessed at Genbank, NM_021913.3 and transcript variant 2 may be accessed at NM_001699.4. The polypeptide sequence of the native protein is provided as SEQ ID NO:1, and specific reference may be made to the sequence with respect to amino acid modifications. Important cellular functions of GAS6/AXL include cell adhesion, migration, phagocytosis, and inhibition of apoptosis. GAS6 and AXL family receptors are highly regulated in a tissue and disease specific manner.
AXL, MER and Tyro3 are each characterized by a unique molecular structure, in that the intracellular region has the typical structure of a receptor tyrosine kinase and the extracellular domain contains fibronectin III and Ig motifs similar to cadherin-type adhesion molecules. During development, AXL, MER and Tyro3 are expressed in various organs, including the brain, suggesting that this RTK is involved in mesenchymal and neural development. In the adult, AXL, MER and Tyro3 expression is low but returns to high expression levels in a variety of tumors. GAS6 is, so far, the single, activating ligand for AXL, MER and Tyro3.
Receptor tyrosine kinases (RTK) are generally activated by ligands that promote receptor dimerization and, in turn, autophosphorylation of tyrosine residues within the cytosolic domain. Binding of signaling proteins to these phosphorylated tyrosine residues then leads to downstream signaling. AXL, MER and Tyro3 family of RTKs are unique in that they are activated by GAS6, members of the vitamin K-dependent protein family that resembles blood coagulation factors rather than typical growth factors.